XRT7250 datasheet(158/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

139

new DS3 frame, but is rather driven by the Terminal

Equipment. Hence, the XRT7250 is referred to as

the Frame Slave.

2. If the user opts to configure the XRT7250 to oper-

ate in Mode 2, it is imperative that the

Tx_Start_of_Frame (or TxFrameRef) signal is syn-

chronized to the TxInClk input clock signal.

Finally, the XRT7250 will pulse its TxOH_Ind output

pin, one bit-period prior to it processing a given over-

head bit, within the outbound DS3 frame. Since the

TxOH_Ind output pin of the XRT7250 is electrically

connected to the DS3_Overhead_Ind, whenever the

XRT7250 pulses the TxOH_Ind output pin "High", it

will also be driving the DS3_Overhead_Ind input pin

(of the Terminal Equipment) "High". Whenever the

Terminal Equipment detects this pin toggling "High", it

should delay transmission of the very next DS3 frame

payload bit by one clock cycle.

The behavior of the signals between the XRT7250

and the Terminal Equipment for DS3 Mode 2 Opera-

tion is illustrated in Figure 47.

How to configure the XRT7250 to operate in this

mode.

1. Set the NibIntf input pin "Low".

2. Set the TimRefSel[1:0] bit-fields (within the

Framer Operating Mode Register) to "01" as

depicted below.

3. Interface the XRT7250, to the Terminal Equip-

ment, as illustrated in Figure 46.

4.2.1.3

Mode 3 - The Serial/Local-Timed/

Frame-Master Mode Behavior of the XRT7250

If the XRT7250 has been configured to operate in this

mode, then the XRT7250 will function as follows.

A. Local Timing - (Uses the TxInClk signal as the

Timing Reference)

FIGURE 47. BEHAVIOR OF THE TERMINAL INTERFACE SIGNALS BETWEEN THE XRT7250 AND THE TERMINAL EQUIP-

MENT

(MODE 2 OPERATION)

Terminal Equipment Signals

DS3_Clock_In

DS3_Data_Out

Tx_Start_of_Frame

DS3_Overhead_Ind

XRT7250 Transmit Payload Data I/F Signals

TxInClk

TxSer

TxFrameRef

TxOH_Ind

Payload[4702]

Payload[4703]

X-Bit

Payload[1]

Payload[4702]

Payload[4703]

X-Bit

Payload[1]

Note: X-Bit will not be processed by the

Transmit Payload Data Input Interface.

DS3 Frame Number N

DS3 Frame Number N + 1

Note: TxFrame pulses high to denote

DS3 Frame Boundary.

Note: TxOH_Ind pulses high to

denote Overhead Data

(e.g., the X-bit).

FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT2

BIT 1

BIT 0

Local Loopback

DS3/E3*

Internal LOS

Enable

RESET

Interrupt

Enable Reset

Frame Format

TimRefSel[1:0]

R/W

101

XRT7250 Datasheet(PDF) 158 Page - Exar Corporation

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